Lightguide fiber of the type used to carry communication signals is fabricated by heating and drawing a portion of a lightguide preform comprised of a refractive core surrounded by glass cladding. One process which has proven extremely useful for the fabrication of preforms is the modified chemical vapor deposition technique (MCVD) whereby reactant-containing precursor gases, such as silicon tetrachloride and germanium tetrachloride are passed through the center of a starter tube. As the starter tube is heated and rotated, reaction by-products, in the form of submicron-sized doped glass particles, are deposited on the inside surface of the tube to form the refractive core of the preform. For a further, more detailed description of the MCVD process, reference should be had to U.S. Pat. No. 4,217,027 issued to J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories.
Other chemical vapor deposition methods such as the Vapor Phase Axial Deposition (VAD) and the Vapor Phase Radial Deposition (VRD) Techniques are presently being investigated to determine their suitability as potential processes for fabricating lightguide preforms. The VAD and the VRD techniques are both characterized by the deposit of submicron-sized doped glass particles on a bait rod to form a boule. Since the submicron-sized doped glass particles deposited on the bait rod during VAD or VRD processes appear as tiny particles of soot, the term "soot deposition" is often used to describe these two techniques. For the same reason, the boule fabricated by the VAD or VRD process is often referred to as a soot boule.
To fabricate a preform from the soot boule, the boule is first consolidated by sintering. Once sintered, the boule is then cladded by insertion into a silica glass tube. In certain instances, the soot boule may be partially cladded by depositing pure silicon dioxide thereon prior to sintering thereof.
Successful implementation of either the VAD or VRD process on a widespread basis is dependent on the quality of the preforms produced thereby. One useful technique for verifying the quality of the preform is to determine the characteristics, particularly the density and the composition, of the soot boule from which the preform is produced. Since the soot particles comprising the unconsolidated boule are generally opaque to visible light, profiling techniques which rely on the use of visible light such as that disclosed in U.S. Pat. No. 4,227,806 issued to L. S. Watkins on Oct. 14, 1980 and assigned to the assignee of the present invention, are not applicable. At present, analysis of soot boules to determine their suitability for use in the fabrication of lightguide preforms is accomplished by slicing the boule and microscopically examining the cross section thereof. Such an analysis technique renders the boule unsuited for later use.
A recent paper "Method of Inspecting Optical Fiber Preforms Using X-ray Absorption Measurements" by H. Takahashi, I. Nakamura, T. Tadatani and T. Kurowa, published in the Japanese Journal Furukawa Denko Jiho 68, pages 143-149 suggests x-ray absorption techniques used in tomographic applications, can be applied to measure the germanium concentration of preforms. However, the measurement method described in the above-referred paper is believed to be flawed since the authors do not appear to account for the possibility of having an inhomogeneous preform because they fail to disclose any method for measuring the density thereof. Consequently, the accuracy of the germanium concentration measurement obtained by this method is doubtful.
Accordingly, there is a need for a method for accomplishing nondestructive analysis of a soot boule.